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CHAPTER 3

CHAPTER 3. Physical Layer. Announcements and Outline. Recap Physical Layer 3.1 Circuits 3.2 Media Outline Physical Layer 3.3 Digital Transmission (Digital Data) 3.4 Analog Transmission (Digital Data) 3.5 Digital Transmission (Analog Data). 2. 3.2 Media Summary. 3.

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CHAPTER 3

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  1. CHAPTER 3 Physical Layer

  2. Announcements and Outline • Recap • Physical Layer 3.1 Circuits 3.2 Media • Outline • Physical Layer 3.3 Digital Transmission (Digital Data) 3.4 Analog Transmission (Digital Data) 3.5 Digital Transmission (Analog Data) 2

  3. 3.2 Media Summary 3

  4. 3.3 Digital Transmission of Digital Data • Computers produce binary data (0 or 1) (i.e. discrete, predictable values) • Standards needed to ensure both sender and receiver understands this data • Codes • Signals 4

  5. 3.3 Coding a character  is represented by a group of bits Letters (A, B, ..), numbers (1, 2,..), special symbols (#, $, ..) 1000001 • ASCII: American Standard Code for Information Interchange • Originally used a 7-bit code (128 combinations), but an 8-bit version (256 combinations) is now in use • Found on PC computers 5

  6. 3.3 Transmission Modes • Bits in a message can be sent on: • a single wire one after another (Serial transmission) • multiple wires simultaneously (Parallel transmission) • Two Modes of transmission: • Serial Mode • Parallel Mode 6

  7. 3.3 Parallel Transmission Example (8 separate copper wires)

  8. 3.3 Serial Transmission Example 8

  9. 3.3 Signaling of Bits • Digital Transmission • Signals sent as a series of “square waves” of either positive or negative voltage • Voltages vary between +3/-3 and +24/-24 depending on the circuit • Signaling (encoding) • Defines how the voltage levels will correspond to the bit values of 0 or 1 • Examples: • Unipolar, Bipolar • RTZ, NRZ, Manchester • Data rate: describes how often the sender can transmit data • 64 Kbps  once every 1/64000 of a second 9

  10. 3.3 Signaling (Encoding) Techniques • Unipolar signaling • Use voltages either vary between 0 and a positive value or between 0 and some negative value • Bipolar signaling • Use both positive and negative voltages • Experiences fewer errors than unipolar signaling • Signals are more distinct (more difficult for interference to change polarity of a current) • Return to zero (RZ) • Signal returns to 0 voltage level after sending a bit • Non return to zero (NRZ) • Signals maintains its voltage at the end of a bit 10

  11. 3.3 Digital Transmission Types UNIPOLAR 11

  12. 3.3 Digital Transmission Types BIPOLAR (RZ) BIPOLAR (NRZ) 12

  13. 3.4 Analog Transmission of Digital Data • A well known example using phone lines to connect PCs to the Internet • PCs generate digital data • Local loop phone lines use analog transmission technology • Modems translate digital data into analog signals 13

  14. 3.4 Telephone Network • Originally designed for human speech (analog communications) only • POTS (Plain Old Telephone Service) • Enables voice communications between two telephones • Human voice (sound waves) converted to electrical signals by the sending telephone • Signals travel through POTS and converted back to sound waves at far end • Sending digital data over POTS • Use modems to convert digital data to an analog format • One modem used by sender to produce analog data • Another modem used by receiver to regenerate digital data 14

  15. 90o 360o 180o 0o 270o 3.4 Sound Waves and Characteristics • Amplitude • Height (loudness) of the wave • Measured in decibels (dB) • Frequency: • Number of waves that pass in a second • Measured in Hertz (cycles/second) • Wavelength, the length of the wave from crest to crest, is related to frequency • Phase: • Refers to the point in each wave cycle at which the wave begins (measured in degrees) • (For example, changing a wave’s cycle from crest to trough corresponds to a 180 degree phase shift). 15

  16. 3.4 Modulation • Μodification of a carrier wave’s fundamental characteristics in order to encode information • Carrier wave: Basic sound wave transmitted through the circuit (provides a base which we can deviate) • Basic ways to modulate a carrier wave: • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM) 16

  17. 3.4 Amplitude Modulation (AM) • Changing the height of the wave to encode data • More susceptible to noise than the other modulation methods 17

  18. 3.4 Frequency Modulation (FM) • Changing the frequency of carrier wave to encode data 18

  19. 3.4 Phase Modulation (PM) • Changing the phase of the carrier wave to encode data 19

  20. 3.4 Concept of Symbol • Symbol: Use each modification of the carrier wave to encode information • Sending one bit of information at a time • One bit encoded for each symbol (carrier wave change)  1 bit per symbol • Sending multiple bits simultaneously • Multiple bits encoded for each symbol (carrier wave change)  n bits per symbol, n > 1 • Need more complicated information coding schemes 20

  21. 3.4 Sending Multiple Bits per Symbol • Possible number of symbols must be increased • 1 bit of information  2 symbols • 2 bits of information  4 symbols • 3 bits of information 8  symbols • 4 bits of information  16 symbols • n bits of information  2n symbols • Multiple bits per symbol might be encoded using amplitude, frequency, and phase modulation • e.g., PM: phase shifts of 0o, 90o, 180o, and 270o • Subject to limitations 21

  22. 3.4 Example: Two-bit AM 4 symbols 22

  23. 3.4 Bit Rate vs. Baud Rate or Symbol Rate Example: AM Example: AM/PM • Bit: a unit of information • Baud: a unit of signaling speed • Bit rate (or data rate): b • Number of bits transmitted per second • Baud rate or symbol rate: s • number of symbols transmitted per second • General formula: b = s x n where b = Data Rate (bits/second) s = Symbol Rate (symbols/sec.) n = Number of bits per symbol 23

  24. 3.5 Digital Transmission of Analog Data • Analog voice data sent over digital network using digital transmission • Requires a pair of special devices called Codec - Coder/decoder • A device that converts an analog voice signal into digital form • Converts it back to analog data at the receiving end • Used by the phone system • Modem is reverse device than Codec, and this word stands for Modulate/Demodulate. Modems are used for analog transmission of digital data. 24

  25. 3.5 Analog to Digital Conversion • Analog data must be translated into a series of bits before transmission onto a digital circuit • Done by a technique called Pulse Amplitude Modulation (PAM) involving 4 steps: • Take samples of the continuously varying analog signal across time • Measure the amplitude of each signal sample • Encode the amplitude measurement of the signal as binary data that is representative of the sample • Send the discrete, digital data stream of 0’s and 1’s that approximates the original analog signal • Creates a rough (digitized) approximation of original signal • Quantizing error: difference between the original analog signal and the replicated but approximated, digital signal • The more samples taken in time, the less quantizing error 25

  26. PAM – Measuring Signal Original wave 8 pulse amplitudes time • Sample analog waveform across time and measure amplitude of signal • In this example, quantize the samples using only 8 pulse amplitudes or levels for simplicity • Our 8 levels or amplitudes can be depicted digitally by using 0’s and 1’s in a 3-bit code, yielding 23 possible amplitudes 26

  27. PAM – Encoding and Sampling Pulse Amplitudes 000 – PAM Level 1 001 – PAM Level 2 010 – PAM Level 3 011 – PAM Level 4 100 – PAM Level 5 101 – PAM Level 6 110 – PAM Level 7 111 – PAM Level 8 8 pulse amplitudes Digitized signal • For digitizing a voice signal, it is typically 8,000 samples per second and 8 bits per sample • 8,000 samples x 8 bits per sample  64,000 bps transmission rate needed • 8,000 samples then transmitted as a serial stream of 0s and 1s 27

  28. PAM for Telephones 28

  29. Combined Modulation Techniques • Combining AM, FM, and PM on the same circuit • Examples • QAM - Quadrature Amplitude Modulation • A widely used family of encoding schemes • Combine Amplitude and Phase Modulation • A common form: 16-QAM • Uses 8 different phase shifts and 2 different amplitude levels • 16 possible symbols  4 bits/symbol • TCM – Trellis-Coded Modulation • An enhancement of QAM • Can transmit different number of bits on each symbol (6,7,8 or 10 bits per symbol) 29

  30. Implications for Management Digital is better Organizational impact Impact on telecom industry 30

  31. Recap • 3 Physical Layer 3.1 Circuits 3.2 Media 3.3 Digital Transmission (Digital Data) 3.4 Analog Transmission (Digital Data) 3.5 Digital Transmission (Analog Data) 31

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